The field of the invention is that of sonar signal processing methods. The invention relates more particularly to sonar echoes coming from objects that are irradiated by sonar emissions with a ‘comb’ line spectrum composed of spectral lines.
One technique exploiting sonar signals, known from the prior art, consists in emitting a known signal into the medium and in processing the received signals by correlation with signals representing replicas of the emitted signal when it is reflected by a moving target and received by the sonar. These replicas, or Doppler copies, are correlated with the signal actually received so as to distinguish the signal coming from the target from the background noise and from reverberation.
The emission of signals having a line spectrum, in other words a spectrum composed of distinct lines, is furthermore of considerable interest for the detection of fast or slow moving underwater targets in an environment limited by reverberation. Indeed, as long as the Doppler shift of the targets is larger than the width of the elementary lines of the spectrum, it allows the reverberation noise to be reduced by Doppler filtering. Such a Doppler filtering process is notably described in the European Patent EP1078280 that relates to a method for detecting moving objects by means of an active sonar.
This type of Doppler filtering consists in carrying out a multicopy correlation of the received signal. Multicopy correlation consists in performing, in a known manner, correlation operations of the received signal with various replicas, or Doppler copies, of the emitted signal. Each Doppler copy exhibits the same spectrum as the signal reflected by an object with a given velocity, which is why they are referred to as Doppler copies of the emitted signal.
The correlation operations are carried out simultaneously, or else during the same period of time, such that the exploitation, in active sonar mode, of emissions of line-spectrum signals leads to as many simultaneous correlation operations being performed as doppler filters are used. Consequently, where the number of Doppler copies employed is large, the multicopy correlation processing mode may require a large computing capacity.
Furthermore, it is known that, for signals whose BT product is large, greater than 50 for example, the Doppler filtering operation by correlation can advantageously be performed in the spectral domain. The letters B and T of the BT product respectively denote the emitted signal band B and its duration T. Indeed, transferring into the spectral domain, for example by means of a Fast Fourier Transform (FFT), turns out to be more advantageous, in terms of computational load, than a direct correlation in the time domain.
Nevertheless, when the correlation is performed with several Doppler copies, this embodiment of doppler filtering leads to a computational load that is still far too high. With this in mind, it will be recalled that the increase in computational load implied by multicopy processing is substantially proportional to the number of copies employed. In practice, the number of copies is of the order of several tens, or even several hundreds. The French Patent Application 03 0404042, filed in April 2003 by the Applicant, mentions in this regard a number of doppler copies greater than 200 required for the doppler filtering.